140 ASTRONOMY. 
fying powers, it will be hereafter possible to perceive more and minuter 
details of the moon’s surface (as, for instance, artificial structures) than at 
present: for with these improvements the difficulties and hindrances will 
increase in like proportion. We need only refer to the atmosphere of the 
earth and the borrowed light of the moon. These difficulties are even now 
experienced in the application of the best telescopes, as the moon, of all the 
heavenly bodies, is that for which the highest powers are unsuited. It is by 
means of very accurate and long continued observations that we are to have 
our knowledge of the moon increased. It is only then that better compari- 
sons with the earlier observations, and more accurate conclusions may be 
drawn than now, when only since the time of Lohrmann and Madler the 
moon has been attentively examined with the more improved refractors. 
Posterity will be able to verify changes which appear to be taking place on 
the surface of the moon, and our successors will probably ridicule many of 
our opinions, and reject them as untenable. One fact is certain, however, 
that Selenography (description of the moon) must commence with generall- 
ties and progress to particulars, while Gecgraphy (description of the earth) 
pursues the opposite method. Selenography has the advantage of Geo- 
graphy, as we do not possess so good a general view of the earth as of the 
half of the moon which is visible to us. 
To become most readily acquainted with the mountains, craters, &c., of 
the moon, it will be necessary to examine attentively the moon at the time 
of the first or last quarter, through a telescope of about 40-60 magnifying 
power, and to make constant reference to the lunar maps. During the full 
moon, this, at least to a beginner, is not very satisfactory, as at this time the 
sun stands directly over the centre of the visible moon’s disk, and the shadows 
of the mountains are not seen. In the first and last quarters, however, the 
sun moves above and below the centre of the visible disk, and at this 
time, accordingly, the shadows of the mountains are greatest and most 
evident. 
The Planets Mars, Jupiter, Saturn, and Uranus. 
56. Pl. 8, fig. 18, represents the planet Mars in his not entirely illuminated 
condition, as seen August 16, 1830, by Sir John Herschel, at Slough, with a 
20-foot reflector. We see plainly enough presumptive continents and seas ; 
the first distinguished by their reddish color, characterizing the light of this 
ever red and fiery planet. In contrast with this color, the seas, if we may so 
term them, appear of a greenish hue. These spots cannot always be seen 
with equal distinctness, which is probably owing to the fact of Mars not 
beirg entirely free from an atmosphere. This supposition is confirmed by 
the exhibition of brilliant white spots at the poles of Mars. These spots are 
probably snow, as they vanish when they have been long exposed to the 
sun, and on the other hand are largest on emerging from the long night of 
their polar winter. By observations on the spots, Mars has been found to 
have a period of rotation in 24 hours, 39 minutes, 21 seconds; and the 
inclination of the axis of rotation to the ecliptic, amounts to about 30° 18’. 
140 
